In an AC circuit, which term describes how effectively the circuit uses current to produce useful work?

• A. Voltage
• B. Current
• C. Resistance
• D. Power factor

Answer: (D)

In AC circuits, how effectively the current is used to do useful work is described by the power factor. It’s the ratio of real power, which does the actual work (heating, lighting, motion), to apparent power, which is the product of voltage and current without considering their phase. For a sinusoidal system, power factor equals cos(phi), where phi is the angle between voltage and current.

When the load is purely resistive, voltage and current are in phase, so the power factor is 1 and essentially all the current does useful work. If the circuit has reactance (inductance or capacitance), current lags or leads voltage, creating reactive power that flows back and forth without doing net work, which lowers the power factor. So, power factor captures how much of the current actually contributes to useful work, while voltage, current, or resistance alone don’t measure that efficiency. A lower power factor means more current is required to deliver the same real power, increasing losses in wiring and equipment.